Safety electrical power unit



April 19, 1938. G. F. SCHMITT SAFETY ELECTRICAL POWER UNIT Filed Sept. 17, 1935 ANN Patented Apr. 19, 1938 UNITED STATES PATENT l OFFICE 6 Claims.

' 'Ihis invention relates to a new and useful improvement in anl apparatus or unit for automatically controlling the flow of current through electrical circuits for preventing arcing and sparking when grounding occurs.

The principal object ofthe invention is to pro- `vide a simple and emcient apparatus to allow the transmission of electric current therethrough from a source of supply to an electrically-driven tool or other device or apparatus without diminution of the current iiow but so arranged and constructed that in the event of' the 'grounding of the tool in the line to which the tool is connected therewill be no ilash or arcing to cause body shocks or fires or explosions in adjacent infiammable materials or gases and the circuit to the supply will be instantaneously cut ofl' and cannot again be closed until the defective tool is re placed or the grounding found and repaired.

Briefly and generally considered the apparatus includes the use of electrical devis such as transformerscircuit breakers. vacuum tubes and others which are so arranged, related and connectedthat the instant the tool or other device is grounded the amount of current which can flow in the circuit is immediately limited sufiiciently to prevent the occurrence of a flash or arc which limitation acts instantaneously to actuate the circuit breaker and thereby cut off the flow of current from the source of supply.

'I'he invention, furthermore, includes the provision of a vacuum-control amplifier tube provided with a negative bias so that normally there is no appreciable current in its plate circuit but when a grounding occurs, the connections or devices employed are such that the grid bias in the tube is altered to a positive Vahle which causes a greatly amplified current to flow in the plate circuit which is connected to a coil in a circuit breaker to throw the same open and cause an immediate practical cessation oi' the ow of current tothe tool or other device.

In the operation of the invention I preferably employ a circuit breaker in association with a vacuum tube control device, whether that device is connected to an A. C. or D. C. circuit. The alterations for D. C. operations will later be mentioned but the association of parts illustrated in the drawing and to be presently described are for A. C. circuits.

Further and more specific objects, features, and advantages will more clearly appear from the subjoin'ed specification when taken in oonnection with the accompanying drawing forming IIA operates.

part thereof and which illustrates a present preferred form of the invention.

In the drawing, the single figure is a dagrammatic representation of the unit when used in conjunction with an A. C. source of supply. 5

As shown in the said drawing, the source of supply from a cable or conductor III is connected to a reel or drum II and thence to a switch I2. 'I'his :permits the control device to be readily moved about at will as a unit in a plant or shop 10 and to any desired location. To the other side of the switch I2 there is connected a transformer A which functions as ordinarily and which has a primary coil I3 and a secondary coil Il with a center tap I5. As'will be obvious, there may 15 be any number of secondary coils I4 desired, depending on the number of tools which are to be connected to the unit. The secondary coil I4 is connected to the terminals of a circuit breaker I6 the other side of which is connected to an outlet socket I1 in which a tool ITA, such as an electrically driven drill, is to be plugged. I8

" represents a clip attached to conductor I9 which is preferably wound on a reel 2|0^, said clip being attached to the work piece C on which the tool Said conductor I9 is grounded by conductor 2IIB to the frame of the safety electrical power unit of the claimed invention.

The center tap I5 is connected to a resistor 2I. Said resistor 2l is connected to the primary coil 22 of a transformer B, the other end being grounded to the frame of the unit and thereby connected to the conductor 20B. Obviously the conductor 20B may be omitted provided the work piece 20 is otherwise grounded to the unit. This transformer B is preferably of the iron-core type. 'Ihe secondary 23 of said transformer B is connected to the grid 24 of a. three-electron vacuum tube amplier A for the purpose of controlling the negative voltage on said grid. 'Ihe plate 25 of tube C is connected to one end of a coil 26 of the circuit breaker I6. The resistorZI is provided to limit the amount of current that can be fed back to transformer A while at the same time allowing a sufficient flow of current to energize transformer B.

Between the switch I2 and the transformer primary I3 of transformer A, conductors 21 and 2l lead to opposite ends of primary coil 29 of power pack transformer D. 'I'his transformer D has three secondary coils 3U, 3| and 32. 'I'hese secondary coils may be wound on core of the transformer secondary A if desired. The coil 32 is connected to the terminals of filament 33 of the three-electrode vacuum tube amplifier C. 55

Coil 30 is connected to the terminals of' the lament 34 of a rectifier vacuum tube E while coil I3 is connected to the other electrodes 35 and 36 of said rectifier tube. 'Ihe power pack transformer D is provided to step up the voltage applied to the rectifier tube E and the function of said rectifier tube is for the purpose of changing the alternating current to direct current.

A center tap from secondary coil 30 of power pack transformer D is connected by conductor 31, first to a filter system 38 and thence to the positive side of a voltage divider 40, the filter system preferably comprising two condensers and a choke coil as indicated. Said filter system acts as an auxiliary to the rectifier tube E and its function is to remove ripples in the line voltage. A center tap from coil 3| connects by conductor 39 to and through said filter system 38 to the negative side of said voltage divider 48, and thence to the secondary coil 23 of transformer B. A conductor 4I is preferably adjustably connected to said voltage divider 40 and is connected to a center tap of the coil 32. A conductor 42 also preferably connects the positive side of the voltage divider 40 with the coil 26 of the circuit breaker I6. The function of the voltage divider is to maintain a continuous and full voltage across the terminals of the rectifier vacuum tube E.

In the operation of thelapparatus the tool I4A is plugged into the outlet II and the clip I8 is attached to the work piece 20C. With the switch I2 closed, power is supplied to the tool through transformer coils I3 and I4 and through the circuit breaker IE, the coil 26 of which is in series with the plate circuit 25 of the vacuum amplifier tube C. The grid 24 of this amplifier tube has a normal negative bias which prevents any appreciable current flowing in the plate circuit 25 of said tube C. However, said grid 24 of the tube C is connected in series with the secondary coil 23 of the transformer B, the primary of which is connected in series with the resistor 2| and to the center tap I of the coil I4 of the transformer A. Therefore, whenever any grounding occurs a flow of current is set up from the said ground to the control line I9 and thence through the conductor 20B to the frame of the unit, thence pass,

ing further to coil 22 through the resistor 2I to the center tap I5 of coil I4 on the transformer A.

The flow of current just-described activates the secondary coil 23- which sets up sufficient current in the grid circuit 24 to overbalance the negative bias thereof and make the grid positive which causes a greatly amplified current to flow in the plate circuit 25 of the amplifier tube. This current being in series with the coil 26 in the circuit breaker I6 permits this coil to be energized sufficiently to trip the circuit breaker and open the main supply circuit. All this is done so rapidly that the circuit is opened before a flash or arc has had time to form at the point of grounding. If an attempt is made to close the circuit breaker before the grounding is repaired the coil 26 will immediately trip again and therefore prevent the flow of current to the tool until and unless the leak or defect is found and -repaired.

The filament and grid circuits 33 and 24 respectively of the amplifier tube C are provided with direct current by the relative connections with the rectifier tube E, the terminals 34, 35 and 36 of which are connected as above described to the several coils of the secondary of the power pack transformer D, which is connected through its primary 23 to the main line in front 0f the switch I2. By employing the intervening filter system 38, the ripples and slight unevenness in the rectified current are smoothed out before they reach' the amplifier tube as hereinbefore explained.

From the foregoing it will be apparent that the circuit breaker I6 between the supply of power and the tool circuit is tripped by a coil 2l which is in circuit with a vacuum tube amplifier C, the grid 24 of which has a normally negative bias but which grid circuit is connected to the power line in such manner that any ow therein caused by a ground will change the grid bias from negative to positive and cause instantaneously an ampliiied current to fiow in the trip coil circuit and trip the circuit breaker. The grid of tube C may thus be said to act as a valve which automatically turns the current on or off leading to the circuit breaker when necessity requires.

The apparatus can be adapted to operate on lD. C. lines on an isolated system such as a ship where local current is generated and not grounded. In other cases where ordinary commercial D. C. is supplied, a grounded neutral is provided such as in the Edison three-wire system and if such an adaptation is made it could be effected if the current is put through a motorl generator to isolate it from its original source.

In this case the main line transformer would be dispensed with as well as the transformer coils 22 and 23 for which a relay would be substituted. The power pack transformer would also be eliminated and the rectifier tube and the filter system and probably the voltage divider 40. In such a system the relay mentioned would operate the circuit breaker so that in this case the change would be effected mechanically instead of electrically as in A. C. operation.

From the above description it is obvious that there has been provided a trip coil in circuit with a device which is responsive to abnormal flow of current in the tool circuit to energize the trip `coil and throw the circuit breaker. In the preferred form the device has an amplifier vacuum tube with the aforesaid trip coil in circuit with the plate of said tube and its grid circuit normaly negatively biased and connected to the main supply line in such manner that a grounded current will change the bias from negative to positive and thereby cause current from the plate circuit to flow through thetrip coil and actuate the circuit breaker.

Whilethe invention has been described in detail and with respect to a present preferred form thereof it is not to be limited to such details and forms since many changes and modifications may be made in the invention without departing from its -spirit and scope as is manifest.

The invention claimed is:

1. A safety system for an electrically driven tool adapted to operate on a work piece comprising a self-contained unit carrying an outlet socket in which said tool is to be plugged, a power line including an electric cable connected to be plugged into a source of current supply and to feed current therefrom to said outlet socket, a circuit breaker in said power line to interrupt the current supply to said outlet socket, means to trip the circuit breaker in response to a ground of the tool winding on said tool without producing a spark at the point of contact between said tool and said work piece, comprising a trip line connected to a point of intermediate potential in said power line and grounded to said unit and a second line grounded to said unit and adapted direct metallic connection between said work piece and said point of intermediate potential, said trip line having a total impedance adapted to limit the current which can flow therethrough in response to aground on said tool to a value such that sparking at the pointl of contact between the tool and the work piece is prevented, and means responsive to said current flow in the trip line to trip said circuit breaker.

2. A safety system for an electrically driven tool adapted to operate on a work piece comprising a self -contained unit carrying an outlet socket in which said tool is to be plugged, a power line including an electric cable connected to be plugged into a source of current supply and to feed current therefrom to said outlet socket, a circuit breaker in said power line to interrupt the current supply to said outlet socket, means to trip the circuit breaker in response to a ground of the tool winding on said tool without producing a spark at the point of contact between said tool and said work piece, comprising a trip line connected to a point of intermediate potential in said power line and grounded to said unit and 1 a second line grounded to said unit and adapted to be connected to said'work piece to establish a direct metallic connection between said work piece and said point of intermediate potential, said trip line having a total impedance adapted to limit the current which can ow therethrough, in response to a ground on said tool to a value y such that sparking at tlie point of contact between the tool and the work piece is prevented, a vacuum tube relay connected to 'trip said circuit b eaker, and means responsive to said current flo in said trip line to actuate said vacuumtube relay. g.

3. A safety system for an electrically driven tool to operate on a work piece, comprising a unit carrying an outlet socket in which said tool is tolbe plugged, a transformer having a primary connected to a source of power-and a secondary connected to said'outlet socket by means of av power line, a circuit breaker in said power line, a trip line comprising an impedance connected by a conductor between an intermediate point on said secondary and said work piece whereby an accidental connection from the winding of said tool to said work piece causes current to flow through said impedance at a voltage less than the full line voltage of said power line, and means responsive tothe flow of current in said impedance for vtripping said circuit breaker, said trip line having a total impedance adapted to reduce the current ow therethrough to a value such that sparking at the point of contact of the tool on said workpiece is prevented.

- fi. A safetysysm for an electrically driven tool to operate on a work piece, comprising a unit carrying an outlet socket in which said tool is to be plugged, a transformer having a primary connected to a source oi' power and a secondary connected to said outlet socket by means of a power line, a circuit breaker in said power line.

a trip line comprising an impedance connectedV by a conductor between the mid-point of said secondary and said work piece whereby an accidental connection from the winding of said tool to said work piece causes current to flow through said impedance at a voltage of approximately half of the voltage on said power line, means l responsive to the ow of current in said impedance for tripping said circuit breaker, said trip line having a total impedance adapted to reduce the current `flow therethrough to a value such that sparking at the point of contact of the toolon said work piece is prevented.

5. A safety system for an electrically driven tool to operate on a work piece, comprising a unit carrying an outlet socket in which said tool is to be plugged, a transformer having a primary connected to a source of power and a secondary connected to said outlet socket by means o'f a power line, a circuit breaker in said power line, a trip line comprising an impedance grounded to said unit and connected to an intermediate point on said secondary whereby less than the full line voltage is applied thereto when a ground occurs on either side of said power line. a conductor having a clip to be connected to the work piece, a reel carrying said conductor and having means grounding said conductor to said unit to establish a direct metallic connection between said work piece and said intermediate point, whereby an accidental connection between the winding of said tool and said work piece causes current to flow through said impedance, and means responsive to the flow of current in said impedance for tripping said circuit breaker, said trip line having a total impedance adapted to reduce the current flow therethrough to a value such that sparking at the point of contact between said tool and said work piece is prevented.

6. A safety system for an electrically driven tool to operate on a work piece, comprising a unit carrying an outlet socket in which said tool is to be plugged, a transformer having a primary connected to a source of power and a secondary connectedto said outlet socket by means of a power line, a circuit breaker in said power line, a trip line comprising an impedance grounded to said unit and connected to an intermediate point on said secondary whereby less than the full line voltage is applied thereto when a ground occurs on either side of said power line, a conductor having a clip `to be connected to the work piece,

a reel carrying said conductor and having means grounding said conductor to said unit to establish a direct metallic connection betweensaid work piece and said intermediate point, whereby an accidental connection between the winding of 

